Masao Sako of the University of Pennsylvania was on vacation with his family when he got the news. The Laser Interferometer Gravitational-wave Observatory, or LIGO, had made a fifth detection of gravitational waves, which expand and contract space time.

While the previous four detections were a result of merging black holes, these waves originated from the merger of two neutron stars, collapsed stars that form the densest form of observable matter in the universe, an event that had never before been observed. Unlike black holes, which don’t emit light, neutron stars are more likely to give out an optical signal when they merge, making them easier to locate.

Sako, a member of the Dark Energy Survey and an associate professor of physics and astronomy in Penn’s School of Arts and Sciences, immediately called his grad student Dillon Brout, who was working to get information from LIGO in real time and produce maps predicting roughly where the source of the waves should be in the sky to help his collaborators find it using the Dark Energy Camera, or DECam.

Scientists on the Dark Energy Survey joined forces for this effort with a team of astronomers based at the Harvard-Smithsonian Center for Astrophysics, or CfA, working with observatories around the world to bolster the original data from DECam. Their images captured the flaring and fading over time of a kilonova, an explosion similar to a supernova, but on a smaller scale, that occurs when neutron stars crash into each other, creating heavy radioactive elements.

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This particular kilonova occurred 130 million years ago in a nearby galaxy, generating the first gravitational waves to ever be traced back to a visible source. Unlike supernovae, which can often remain visible in the sky for months after the initial explosion, this kilonova lasted a fraction of that time before disappearing into the noise.

“It’s unlike anything else that we've seen,” Sako said. “Ordinary stellar explosions usually get bright on timescales of a week and take months for it to fade. This thing pretty much disappeared in two weeks.”

Because of the event’s proximity, scientists were able, within a few hours of receiving the notice from LIGO/Virgo, to point telescopes in the direction of the event and get a clear picture of the light.

DECam, the most powerful survey instrument of its kind, was built and tested at Fermilab, the lead laboratory on the Dark Energy Survey, and is mounted on the National Science Foundation’s 4-meter Blanco telescope, part of the Cerro Tololo Inter-American Observatory in Chile, a division of the National Optical Astronomy Observatory. The DES images are processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.

The kilonova was first identified in DECam images by Ohio University astronomer Ryan Chornock, who instantly alerted his colleagues by email.

“I was flipping through the raw data, and I came across this bright galaxy and saw a new source that was not in the reference image taken previously,” he said. “It was very exciting.”

Next, a team led by Edo Berger, from CfA, went to work analyzing the phenomenon using several different resources. Within hours of receiving the location information, the team had booked time with several observatories, including NASA’s Hubble Space Telescope and Chandra X-ray Observatory.

LIGO/Virgo works with dozens of astronomy collaborations around the world, providing sky maps of the area where any detected gravitational waves originated. The team from DES and CfA had been preparing for an event like this for more than two years, forging connections with other astronomy collaborations and putting procedures in place to mobilize as soon as word came down that a new source had been detected. The result is rich data set that covers “radio waves to X-rays to everything in between,” Berger said.

In addition to providing scientists with a new way to probe the physics of mysterious, compact objects such as neutron stars and black holes, this event also provides a new and unique way to measure the present expansion rate of the universe.

In 2006, Bhuvnesh Jain, Walter H. and Leonore C. Annenberg Professor in the Natural Sciences in Penn Arts and Sciences, collaborated on a theoretical paper published in Physical Review D pointing out that an “optical counterpart” to gravitational waves would allow scientists to measure expansion rate of the universe and therefore test for dark energy. 

“One of the papers to be released Monday is indeed an estimate of the expansion rate using this one event,” Jain said. “We had not dreamed that in just over a decade after our theory paper, not just gravity waves, but the accompanying light would be detected. The expansion rate measurements are not accurate yet but with 10 or more such events it will get very interesting.”

Current methods of measuring the expansion rate of the universe involve what’s called a “distance ladder.” By getting an accurate distance of one object, they can use that distance to accurately measure the distance of something else, building to larger and larger distances. But every “rung” in the ladder provides more opportunity for error.

Using kilonovae to measure the expansion of the universe would skip the distance ladder, going straight to a distance measurement which, combined with information about the redshift or recession speed, enables scientists to determine the present expansion rate.

DECam has the capability to observe kilonovae as much as 10 times the distance of this one, which will play an important role in future detections of gravitational waves from merging neutron stars, allowing scientists to locate and image sources at even greater distances. This new kind of measurement will assist the Dark Energy Survey uncover more about dark energy, the mysterious force accelerating the expansion of the universe.

“Marrying optical data with gravitational wave data,” Brout said, “marks the beginning of a new era in astrophysics. It provides a unique opportunity to weigh in on the current debate over the expansion rate of the universe and may even tell us about the nature of dark energy. We’re directly testing our theories of the universe in a completely new and independent way. This first discovery is already beyond our wildest expectations, and it’s hard to imagine what we’ll learn next.”

A paper describing the DECam discovery of the optical counterpart was recently accepted for publication in The Astrophysical Journal. A preprint is available here.

“It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the universe,” said France A. Córdova, director of the National Science Foundation, which funds LIGO and supports the observatory where DECam is housed. “This discovery realizes a long-standing goal many of us have had, that is, to simultaneously observe rare cosmic events using both traditional as well as gravitational-wave observatories. Only through NSF’s four-decade investment in gravitational-wave observatories, coupled with telescopes that observe from radio to gamma-ray wavelengths, are we able to expand our opportunities to detect new cosmic phenomena and piece together a fresh narrative of the physics of stars in their death throes.”

The Dark Energy Survey is a collaboration of more than 400 scientists from 26 institutions in seven countries. Funding for the DES projects has been provided by the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, ETH Zurich for Switzerland, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Center for Cosmology and AstroParticle Physics at Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, the Financiadora de Estudos e Projetos, Fundação Carlos the Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, the Conselho Nacional de Desenvolvimento Científico e Tecnológico and Ministério da Ciência e Tecnologia, the Deutsche Forschungsgemeinschaft and the collaborating institutions in the Dark Energy Survey, the list of which can be found at www.darkenergysurvey.org/collaboration.